The present invention relates to an electrolytic device for contaminant removal suitable for use in a liquid chromatographic system.
Suppressed ion chromatography is a known technique for analysis of sample ions of one charge in an eluent containing electrolyte. First, the sample ions in the eluent are chromatographically separated. Then, the eluent is suppressed by removal of the electrolyte counterions to the sample ions, and the sample ions are detected, typically by an electrical conductivity detector. One type of suppressor device, called a sandwich membrane suppressor, is described in U.S. Pat. No. 4,999,098 (the “'098 patent”). In one embodiment, the suppressor includes three channels. During suppression, the eluent and separated sample ions flow through the central channel of the suppressor while regenerant solution flows in the two outside channels. The outside two channels are separated from the central channel by barriers having exchangeable ions capable of passing ions of only one charge, positive or negative, and of blocking bulk liquid flow. Suitable barriers are ion-exchange membranes sold under the trademark Nafion®. One embodiment is an electrolytic, three-channel flat membrane suppressor illustrated in FIGS. 2 and 3 of the '098 patent. For an anion analysis, the eluent including the analyte anions which have been previously separated on a chromatographic column, comprising a packed bed of anion exchange resin, flows through the central channel. The ion-exchange membranes include exchangeable cations. Eluent cations are removed from the central channel and are drawn toward the negative electrode across the adjacent membrane barrier, as illustrated in FIG. 3 of the '098 patent. Thus, if sodium hydroxide is used as the electrolyte of the eluent, the sodium ion is removed from the central channel across the cation exchange membrane adjacent to the cathode. A device of this type has also been used for purposes other than suppression such as pretreatment of a liquid sample prior to chromatographic separation.
An electrolytic device suitable for use in a liquid chromatography system is disclosed in co-pending U.S. application Ser. No. 14/028,064, filed Sep. 16, 2013. The device comprises a housing including at least first, second, third, and fourth side-by-side liquid flow-through channels, each having an inlet and an outlet. The channels are separated from each other by charged membrane barriers having exchangeable ions capable of passing ions of only one charge, positive or negative, and of blocking bulk liquid flow. A first electrode is disposed adjacent to and along the first channel in electrical communication therewith; and a second electrode is disposed adjacent to and along the fourth channel in electrical communication therewith. The device is disclosed for use in a number of applications such as sample stream pretreatment, and eluent suppression.
The presence of ionic contaminants leads to performance problems when pursuing ion chromatography. It can affect the background which can impact the absolute response. For example, for anion analysis when the contaminant is carbon dioxide the background after suppression is carbonic acid, a weak acid which results in low response for all anions. Varying backgrounds can impact the reproducibility of the separation since the analyte response would vary resulting in errors in quantitation. The quantitation of the peak of interest could be compromised by a changing background making it difficult to draw baselines in order to integrate the peaks of interest. Also, contaminant peaks vary in concentration from run to run and day to day. This makes quantitation of analytes of interest very difficult particularly under trace analysis conditions. Overall, inconsistent baselines and blanks can result in poor reproducibility of the analysis and lead to errors in quantitation.
In chromatography systems sold by Dionex Corporation under the tradename RFIC, a continuously regenerated trap column is used for contaminant removal. The device is electrolytically regenerated and uses a DC potential to achieve the contaminant removal. A DC power supply is therefore needed for device operation. The contaminants are removed via an ion exchange membrane. The device can be installed on the high pressure side of an eluent generator or can be installed before an eluent generator but after the pump. In the former placement, the device removes contaminants from the eluent stream which comprises contaminants from deionized water, eluent generation or the pumping process. In the latter placement, the device removes contaminants only from the influent deionized water stream. Other devices such as a deionizer or water purifier can be used to remove the contaminants stemming from the influent deionized stream before the pumping process. These devices are effective only when operated with a flowing stream because storing purified water and retaining it contaminant free is challenging. These devices, however, do not address contaminants stemming from other sources such as storage containers, pump seals, pumps etc.
It has been discovered that when operating a device in the eluent stream, highly retained components on the contaminant removal device, such as a polymeric anions are accumulated. When this occurs, the removal of small ionic contaminant such as carbonate becomes difficult since the capacity of the phase is not fully available for the contaminant removal process. Under these conditions the competing effects of the eluent favors elution of the smaller contaminants which adversely affects chromatographic performance. Using two devices for the contaminant removal function such as two continuously regenerated trap columns, one before the eluent generator and one after, is not desirable as this doubles the costs with the use of two devices and two power supplies.